1. Field of the Invention
The present invention relates to reticles, and more particularly to reticles used in photolithographic systems used for fabricating integrated circuit devices.
2. Description of Related Art
Integrated circuit device fabrication requires that precisely controlled quantities be introduced into or deposited onto tiny regions of a wafer or substrate. Photolithography is typically used to create patterns that define these regions. That is, photoresist is spin-coated onto the wafer, selectively exposed to radiation, and then developed. If positive photoresist is used then the developer removes the irradiated regions, whereas if negative photoresist is used then the developer removes the non-irradiated regions. After the photoresist is patterned, the wafer is subjected to an additive process (such as ion implantation) or a subtractive process (such as etching) using the photoresist as a mask.
Photolithographic systems typically use a radiation source and a lens in conjunction with a mask or reticle to selectively irradiate the photoresist. The radiation source projects radiation through the mask or reticle to the lens, and the lens focuses an image of the mask or reticle onto the wafer. A mask transfers a pattern onto the entire wafer (or another mask) in a single exposure step, whereas a reticle transfers a pattern onto only a portion of the wafer. Step and repeat systems transfer multiple images of the reticle pattern over the entire wafer using multiple exposures. The reticle pattern is typically 2.times. to 10.times. the size of the image on the wafer, due to reduction by the lens. However, non-reduction (1.times.) steppers offer a larger field, thereby allowing more than one pattern to be printed at each exposure.
The reticle is typically composed of quartz with relatively defect-free surfaces and a high optical transmission at the radiation wavelength. Quartz has a low thermal expansion coefficient and high transmission for near and deep ultraviolet light. Although quartz tends to be expensive, it has become more affordable with the development of high quality synthetic quartz material.
The reticle is prepared by cutting a large quartz plate which is polished and cleaned, and then coated with a mask forming material such as chrome or iron oxide. Chrome is the most widely used material and is typically deposited by sputtering or evaporation to a thickness of less than 1,000 angstroms. The chrome is then selectively removed to form the pattern. For instance, a very thin layer of photoresist is deposited on the chrome and patterned (either optically or by an electron beam) by imaging and exposing a set of accurately positioned rectangles, and then a wet etch is applied. Patterning the reticle for a complex VLSI circuit level may require in excess of 100,000 rectangle exposures over a 10 hour period. During this period, extreme temperature control is often necessary to prevent positional errors. As a result, the quality of the reticle cannot be ascertained until after the chrome is etched.
Lens errors in step and repeat systems are highly undesirable since they disrupt the pattern transfer from the reticle to the photoresist, which in turn introduces flaws into the integrated circuit manufacturing process. Lens errors include a variety of optical aberrations, such as astigmatism and distortion. Astigmatism arises when the lens curvature is irregular. Distortion arises when the lens magnification varies with radial distance from the lens center. For instance, with positive or pincushion distortion, each image point is displaced radially outward from the center and the most distant image points are displaced outward the most. With negative or barrel distortion, each image point is displaced radially inward toward the center and the most distant image points are displaced inward the most. Accordingly, the lens error is frequently measured so that corrections or compensations can be made.
A typical technique for evaluating lens errors includes performing a photoresist exposure and development using specially designed mask patterns to be used for evaluation purposes. After such an imaging process, the wafer is either subjected to an optical inspection or is further processed to form electrically measurable patterns. The use of photosensitive detectors fabricated on silicon to monitor optical systems is also known in the art.
U.S. Pat. No. 4,585,342 discloses a silicon wafer with radiation sensitive detectors arranged in a matrix, an x-y stage for positioning the wafer so that each one of the detectors is separately disposed in sequence in the same location in the field of projected radiation, and a computer for recording the output signals of the detectors in order to calibrate the detectors prior to evaluating the performance of an optical lithographic system.
U.S. Pat. No. 5,402,224 provides distortion inspection of an optical system by providing a test reticle with a measurement pattern arranged at a predetermined interval Sx, transferring a the pattern from a test reticle to a photosensitive substrate, shifting the test reticle and the substrate relative to one another by .DELTA.Tx (where .DELTA.Tx&lt;Sx), transferring measurement pattern again from the test reticle to the substrate, measuring the relative displacement between the two formed patterns to provide differential coefficients on distortion characteristics, and integrating the differential coefficients to provide the distortion characteristics.
Replacing the lens in a step and repeat system is considered impractical since the lens is a large, heavy, integral part of the system. Furthermore, it is unlikely that a substitute lens will render subsequent corrections unnecessary.
U.S. Pat. No. 5,308,991 describes predistorted reticles which incorporate compensating corrections for known lens distortions. Lens distortion data is obtained which represents the feature displacement on a wafer as a function of the field position of the lens. The lens distortion data is used to calculate x and y dimensional corrections terms. The inverted correction terms are multiplied by a stage controller's compensation value to correctly position the reticle. In this manner, the reticle is positioned to compensate for the lens error. A drawback to this approach, however, is that a highly accurate reticle positioning apparatus is required.
Accordingly, a need exists for an improved technique that compensates for lens errors in photolithographic systems.